Fix multi-file usage

1. Follow aliases with getSupers.
2. Make fix in file with superclass, not with the error.

src/services/codefixes/fixPropertyOverrideAccessor.ts

@@ -47,6 +47,7 @@ namespace ts.codefix {
 
             startPosition = baseProp.valueDeclaration.pos;
             endPosition = baseProp.valueDeclaration.end;
+            file = getSourceFileOfNode(baseProp.valueDeclaration);
         }
         else {
             Debug.fail("fixPropertyOverrideAccessor codefix got unexpected error code " + code);

src/services/codefixes/generateAccessors.ts

@@ -229,8 +229,11 @@ namespace ts.codefix {
         while (decl) {
             const superElement = getClassExtendsHeritageElement(decl);
             const superSymbol = superElement && checker.getSymbolAtLocation(superElement.expression);
-            const superDecl = superSymbol && find(superSymbol.declarations, isClassLike);
-            if (superDecl) { res.push(superDecl); }
+            if (!superSymbol) break;
+            const symbol = superSymbol.flags & SymbolFlags.Alias ? checker.getAliasedSymbol(superSymbol) : superSymbol;
+            const superDecl = find(symbol.declarations, isClassLike);
+            if (!superDecl) break;
+            res.push(superDecl);
             decl = superDecl;
         }
         return res;

tests/cases/fourslash/codeFixPropertyOverrideAccess3.ts

@@ -0,0 +1,28 @@
+/// <reference path='fourslash.ts' />
+
+// @strict: true
+
+// @Filename: foo.ts
+//// import { A } from './source'
+//// class B extends A {
+////     get x() { return 2 }
+//// }
+// @Filename: source.ts
+//// export class A {
+////     x = 1
+//// }
+
+verify.codeFix({
+    description: `Generate 'get' and 'set' accessors`,
+    newFileContent: {
+        '/tests/cases/fourslash/source.ts': `export class A {
+    private _x = 1;
+    public get x() {
+        return this._x;
+    }
+    public set x(value) {
+        this._x = value;
+    }
+}`},
+    index: 0
+})