Add optional message to assert in Dart2js.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index fbcbb0f49d601ea04a8f45557114bdc9e1c390ff..edce918b67714f55ed4bea8a5d388163a0e34ef7 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -278,7 +278,9 @@ class JavaScriptBackend extends Backend {
 
   ClassElement closureClass;
   ClassElement boundClosureClass;
-  Element assertMethod;
+  Element assertTestMethod;
+  Element assertThrowMethod;
+  Element assertHelperMethod;
   Element invokeOnMethod;
 
   ClassElement jsInterceptorClass;
@@ -1354,8 +1356,6 @@ class JavaScriptBackend extends Backend {
     }
   }
 
-  bool isAssertMethod(Element element) => element == assertMethod;
-
   void registerRequiredType(DartType type, Element enclosingElement) {
     // If [argument] has type variables or is a type variable, this method
     // registers a RTI dependency between the class where the type variable is
@@ -2129,7 +2129,9 @@ class JavaScriptBackend extends Backend {
         jsMutableIndexableClass = findClass('JSMutableIndexable');
       } else if (uri == DART_JS_HELPER) {
         initializeHelperClasses();
-        assertMethod = findHelper('assertHelper');
+        assertTestMethod = findHelper('assertTest');
+        assertThrowMethod = findHelper('assertThrow');
+        assertHelperMethod = findHelper('assertHelper');
 
         typeLiteralClass = findClass('TypeImpl');
         constMapLiteralClass = findClass('ConstantMap');
@@ -2926,8 +2928,13 @@ class JavaScriptResolutionCallbacks extends ResolutionCallbacks {
     registry.registerInstantiation(element.rawType);
   }
 
-  void onAssert(Send node, Registry registry) {
-    registerBackendStaticInvocation(backend.assertMethod, registry);
+  void onAssert(bool hasMessage, Registry registry) {
+    if (hasMessage) {
+      registerBackendStaticInvocation(backend.assertTestMethod, registry);
+      registerBackendStaticInvocation(backend.assertThrowMethod, registry);
+    } else {
+      registerBackendStaticInvocation(backend.assertHelperMethod, registry);
+    }
   }
 
   void onAsyncForIn(AsyncForIn node, Registry registry) {