Better toString and Error.toSafeString output for closures for dart2js.

pkg/compiler/lib/src/js_emitter/class_stub_generator.dart

Index: pkg/compiler/lib/src/js_emitter/class_stub_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/class_stub_generator.dart b/pkg/compiler/lib/src/js_emitter/class_stub_generator.dart
index aef381f2c88d1bc67b31354f4e521bfde8703691..e21ba1daf0bd280ce3c6192fe7e634dee5d0c232 100644
--- a/pkg/compiler/lib/src/js_emitter/class_stub_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/class_stub_generator.dart
@@ -224,7 +224,7 @@ List<jsAst.Statement> buildTearOffCode(JavaScriptBackend backend) {
   } else {
     // Default values for mocked-up test libraries.
     tearOffAccessText =
-        r'''function() { throw 'Helper \'closureFromTearOff\' missing.' }''';
+        r'''function() { throw "Helper 'closureFromTearOff' missing." }''';
     tearOffAccessExpression = js(tearOffAccessText);
     tearOffGlobalObjectName = 'MissingHelperFunction';
     tearOffGlobalObject = '($tearOffAccessText())';