add mergeFromJsonMap and writeToJsonMap to GeneratedMessage

These methods were requested by users who want to do JSON
encoding and decoding themselves.

Also added basic smoke tests for the JSON methods.

lib/src/protobuf/generated_message.dart

@@ -630,7 +630,12 @@ abstract class GeneratedMessage {
 
   // JSON support.
 
-  Map<String, dynamic> _toMap() {
+  /**
+   * Returns the JSON encoding of this message as a Dart map.
+   *
+   * The encoding is described in [GeneratedMessage.writeToJson].
+   */
+  Map<String, dynamic> writeToJsonMap() {
     convertToMap(fieldValue, fieldType) {
       int scalarType = fieldType & ~(_REPEATED_BIT | _PACKED_BIT);
 
@@ -666,7 +671,7 @@ abstract class GeneratedMessage {
         return fieldValue.toString();
       case _GROUP_BIT:
       case _MESSAGE_BIT:
-        return fieldValue._toMap();
+        return fieldValue.writeToJsonMap();
       default:
         throw 'Unknown type $fieldType';
       }
@@ -684,18 +689,19 @@ abstract class GeneratedMessage {
   }
 
   /**
-   * Return a JSON string that encodes this message.  Each message (top level
-   * or nested) is represented as an object delimited by curly braces.  Within
-   * a message, elements are indexed by tag number (surrounded by quotes).
-   * Repeated elements are represented as arrays.
+   * Returns a JSON string that encodes this message.
+   * 
+   * Each message (top level or nested) is represented as an object delimited
+   * by curly braces.  Within a message, elements are indexed by tag number
+   * (surrounded by quotes). Repeated elements are represented as arrays.
    *
    * Boolean values, strings, and floating-point values are represented as
    * literals.  Values with a 32-bit integer datatype are represented as integer
    * literals; values with a 64-bit integer datatype (regardless of their
    * actual runtime value) are represented as strings.  Enumerated values are
    * represented as their integer value.
    */
-  String writeToJson() => JSON.encode(_toMap());
+  String writeToJson() => JSON.encode(writeToJsonMap());
 
   // Merge fields from a previously decoded JSON object.
   void _mergeFromJson(
@@ -840,7 +846,7 @@ abstract class GeneratedMessage {
 
 
   /**
-   * Merge field values from a JSON object, encoded as described by
+   * Merges field values from a JSON object, encoded as described by
    * [GeneratedMessage.writeToJson].
    */
   void mergeFromJson(
@@ -849,6 +855,17 @@ abstract class GeneratedMessage {
     _mergeFromJson(JSON.decode(data), extensionRegistry);
   }
 
+  /**
+   * Merges field values from a JSON object represented as a Dart map.
+   * 
+   * The encoding is described in [GeneratedMessage.writeToJson].
+   */
+  void mergeFromJsonMap(
+      Map<String, dynamic> json,
+      [ExtensionRegistry extensionRegistry = ExtensionRegistry.EMPTY]) {
+    _mergeFromJson(json, extensionRegistry);
+  }
+
   /**
    * Add an extension field value to a repeated field.  The backing
    * [List] will be created if necessary.

test/all_tests.dart

@@ -7,10 +7,12 @@ library protobuf_lib_all_tests;
 
 import 'codec_test.dart' as ct;
 import 'coded_buffer_reader_test.dart' as cbrt;
+import 'json_test.dart' as jt;
 import 'pb_list_test.dart' as plt;
 
 void main() {
   ct.main();
   cbrt.main();
+  jt.main();
   plt.main();
 }

test/json_test.dart

@@ -0,0 +1,86 @@
+// Basic smoke tests for the GeneratedMessage JSON API.
+//
+// There are more JSON tests in the dart-protoc-plugin package.
+library json_test;
+
+import 'dart:convert';
+import 'package:protobuf/protobuf.dart';
+import 'package:unittest/unittest.dart';
+
+main() {
+  T example = new T()
+    ..a = 123
+    ..b = "hello";
+
+  test('testWriteToJson', () {
+    String json = example.writeToJson();
+    checkJsonMap(JSON.decode(json));
+  });
+
+  test('writeToJsonMap', () {
+    Map m = example.writeToJsonMap();
+    checkJsonMap(m);
+  });
+
+  test('testMergeFromJson', () {
+    var t = new T();
+    t.mergeFromJson('''{"1": 123, "2": "hello"}''');
+    checkMessage(t);
+  });
+
+  test('testMergeFromJsonMap', () {
+    var t = new T();
+    t.mergeFromJsonMap({"1": 123, "2": "hello"});
+    checkMessage(t);
+  });
+}
+
+checkJsonMap(Map m) {
+  expect(m.length, 2);
+  expect(m["1"], 123);
+  expect(m["2"], "hello");
+}
+
+checkMessage(T t) {
+  expect(t.a, 123);
+  expect(t.b, "hello");
+}
+
+/// This is generated code based on:
+/// dart-protoc-plugin/test/protos/toplevel.proto
+/// (Copied to avoid a circular dependency.)
+///
+/// message T {
+///   optional int32 a = 1;
+///   optional string b = 2;
+/// }
+class T extends GeneratedMessage {
+  static final BuilderInfo _i = new BuilderInfo('T')
+    ..a(1, 'a', GeneratedMessage.O3)
+    ..a(2, 'b', GeneratedMessage.OS)
+    ..hasRequiredFields = false;
+
+  T() : super();
+  T.fromBuffer(List<int> i, [ExtensionRegistry r = ExtensionRegistry.EMPTY])
+      : super.fromBuffer(i, r);
+  T.fromJson(String i, [ExtensionRegistry r = ExtensionRegistry.EMPTY])
+      : super.fromJson(i, r);
+  T clone() => new T()..mergeFromMessage(this);
+  BuilderInfo get info_ => _i;
+  static T create() => new T();
+  static PbList<T> createRepeated() => new PbList<T>();
+
+  int get a => getField(1);
+  void set a(int v) {
+    setField(1, v);
+  }
+  bool hasA() => hasField(1);
+  void clearA() => clearField(1);
+
+  String get b => getField(2);
+  void set b(String v) {
+    setField(2, v);
+  }
+  bool hasB() => hasField(2);
+  void clearB() => clearField(2);
+}