Add optional message to assert in Dart2js.

pkg/compiler/lib/src/typechecker.dart

Index: pkg/compiler/lib/src/typechecker.dart
diff --git a/pkg/compiler/lib/src/typechecker.dart b/pkg/compiler/lib/src/typechecker.dart
index b72eab124dacfcab5c1c9cb37cdf6541a74e108b..6f5791c8f62ff624b7c89216d6aea69e09e898da 100644
--- a/pkg/compiler/lib/src/typechecker.dart
+++ b/pkg/compiler/lib/src/typechecker.dart
@@ -603,6 +603,12 @@ class TypeCheckerVisitor extends Visitor<DartType> {
     return type;
   }
 
+  DartType visitAssert(Assert node) {
+    analyze(node.condition);
+    if (node.hasMessage) analyze(node.message);
+    return const StatementType();
+  }
+
   DartType visitBlock(Block node) {
     return analyze(node.statements);
   }
@@ -1171,10 +1177,6 @@ class TypeCheckerVisitor extends Visitor<DartType> {
   }
 
   DartType visitSend(Send node) {
-    if (elements.isAssert(node)) {
-      return analyzeInvocation(node, const AssertAccess());
-    }
-
     Element element = elements[node];
 
     if (element != null && element.isConstructor) {