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 b0f9a82df29609e06f49cf76954fd248cbdd26b8..44d82ea35e99411e149020061661bf310352541a 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -280,7 +280,8 @@ class JavaScriptBackend extends Backend {
 
   ClassElement closureClass;
   ClassElement boundClosureClass;
-  Element assertMethod;
+  Element assertConditionHelperMethod;
+  Element assertThrowMethod;
   Element invokeOnMethod;
 
   ClassElement jsInterceptorClass;
@@ -1332,8 +1333,6 @@ 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
@@ -2108,7 +2107,8 @@ class JavaScriptBackend extends Backend {
         jsMutableIndexableClass = findClass('JSMutableIndexable');
       } else if (uri == DART_JS_HELPER) {
         initializeHelperClasses();
-        assertMethod = findHelper('assertHelper');
+        assertConditionHelperMethod = findHelper('assertConditionHelper');
+        assertThrowMethod = findHelper('assertThrowHelper');
 
         typeLiteralClass = findClass('TypeImpl');
         constMapLiteralClass = findClass('ConstantMap');
@@ -2888,8 +2888,11 @@ class JavaScriptResolutionCallbacks extends ResolutionCallbacks {
     registry.registerInstantiation(element.rawType);
   }
 
-  void onAssert(Send node, Registry registry) {
-    registerBackendStaticInvocation(backend.assertMethod, registry);
+  void onAssert(Registry registry) {
+    registerBackendStaticInvocation(backend.assertConditionHelperMethod,
+                                    registry);
+    registerBackendStaticInvocation(backend.assertThrowMethod,
+                                    registry);
   }
 
   void onAsyncForIn(AsyncForIn node, Registry registry) {