Add --version option to dart2js and add version information (if available) to generated code.

sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
index 6ecbfd5099e1e4ab723fd545a494966e5544cb0b..d6c1b6062225e31c36744108514c8151aa490f45 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart
@@ -2948,7 +2948,7 @@ if (typeof document !== "undefined" && document.readyState !== "complete") {
 
       computeNeededClasses();
 
-      mainBuffer.add(GENERATED_BY);
+      mainBuffer.add(buildGeneratedBy());
       addComment(HOOKS_API_USAGE, mainBuffer);
       mainBuffer.add('function ${namer.isolateName}()$_{}\n');
       mainBuffer.add('init()$N$n');
@@ -3173,7 +3173,7 @@ if (typeof document !== "undefined" && document.readyState !== "complete") {
   void emitDeferredPreambleWhenEmpty(CodeBuffer buffer) {
     if (!buffer.isEmpty) return;
 
-    buffer.write(GENERATED_BY);
+    buffer.write(buildGeneratedBy());
 
     buffer.write('var old${namer.CURRENT_ISOLATE}$_='
                  '$_${namer.CURRENT_ISOLATE}$N');
@@ -3186,6 +3186,12 @@ if (typeof document !== "undefined" && document.readyState !== "complete") {
                  '$_${namer.isolateName}.prototype$N$n');
   }
 
+  String buildGeneratedBy() {
+    var suffix = '';
+    if (BUILD_ID != null) suffix = ' version: $BUILD_ID';
+    return '// Generated by dart2js, the Dart to JavaScript compiler$suffix.\n';
+  }
+
   String buildSourceMap(CodeBuffer buffer, SourceFile compiledFile) {
     SourceMapBuilder sourceMapBuilder =
         new SourceMapBuilder(compiler.sourceMapUri);
@@ -3214,10 +3220,6 @@ if (typeof document !== "undefined" && document.readyState !== "complete") {
   }
 }
 
-const String GENERATED_BY = """
-// Generated by dart2js, the Dart to JavaScript compiler.
-""";
-
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called