Add --generate-js-only flag to test runner

This will return early right after generating JS from the wast test
files. It will not attempt to run the tests, or do the round trip
conversion from wasm <-> wast.

This is convenient for proposals to add tests without having to
update the reference interpreter with implementation, and generate those
tests to JS to run in other Wasm engines.

Fixes WebAssembly#1430.

test/core/run.py

@@ -17,6 +17,7 @@
 parser = argparse.ArgumentParser()
 parser.add_argument("--wasm", metavar="<wasm-command>", default=os.path.join(os.getcwd(), "wasm"))
 parser.add_argument("--js", metavar="<js-command>")
+parser.add_argument("--generate-js-only", action='store_true')
 parser.add_argument("--out", metavar="<out-dir>", default=outputDir)
 parser.add_argument("file", nargs='*')
 arguments = parser.parse_args()
@@ -28,6 +29,7 @@
 
 wasmCommand = arguments.wasm
 jsCommand = arguments.js
+generateJsOnly = arguments.generate_js_only
 outputDir = arguments.out
 inputFiles = arguments.file if arguments.file else main_test_files + simd_test_files
 
@@ -64,6 +66,14 @@ def _runTestFile(self, inputPath):
     dir, inputFile = os.path.split(inputPath)
     outputPath = os.path.join(outputDir, inputFile)
 
+    # Generate JS first, then return early if we are only generating JS.
+    jsPath = self._auxFile(outputPath.replace(".wast", ".js"))
+    logPath = self._auxFile(jsPath + ".log")
+    self._runCommand(('%s -d "%s" -o "%s"') % (wasmCommand, inputPath, jsPath), logPath)
+
+    if generateJsOnly:
+      return
+
     # Run original file
     expectedExitCode = 1 if ".fail." in inputFile else 0
     logPath = self._auxFile(outputPath + ".log")
@@ -96,9 +106,6 @@ def _runTestFile(self, inputPath):
     self._runCommand(('%s -d "%s" -o "%s"') % (wasmCommand, wasm2Path, wast2Path), logPath)
     self._compareFile(wastPath, wast2Path)
 
-    jsPath = self._auxFile(outputPath.replace(".wast", ".js"))
-    logPath = self._auxFile(jsPath + ".log")
-    self._runCommand(('%s -d "%s" -o "%s"') % (wasmCommand, inputPath, jsPath), logPath)
     if jsCommand != None:
       self._runCommand(('%s "%s"') % (jsCommand, jsPath), logPath)