Support enums in dart2js.

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index abd5e29594869b5486b149f52854b583ddcb859b..b58554e2c765ff0500af09ae1c73d24d5025a5cf 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -1492,7 +1492,8 @@ class SsaBuilder extends ResolvedVisitor {
   HGraph buildLazyInitializer(VariableElement variable) {
     ast.Node node = variable.node;
     openFunction(variable, node);
-    assert(variable.initializer != null);
+    assert(invariant(variable, variable.initializer != null,
+        message: "Non-constant variable $variable has no initializer."));
     visit(variable.initializer);
     HInstruction value = pop();
     value = potentiallyCheckType(value, variable.type);