Quick work around dart2js crashes

dart/pkg/compiler/lib/src/compile_time_constants.dart

Index: dart/pkg/compiler/lib/src/compile_time_constants.dart
diff --git a/dart/pkg/compiler/lib/src/compile_time_constants.dart b/dart/pkg/compiler/lib/src/compile_time_constants.dart
index aa893c35bd4a2cc2d17396c2c61bb4dff6b8ef55..6397242081cf1f897e56d1ee6360b5d507c32201 100644
--- a/dart/pkg/compiler/lib/src/compile_time_constants.dart
+++ b/dart/pkg/compiler/lib/src/compile_time_constants.dart
@@ -610,6 +610,8 @@ class CompileTimeConstantEvaluator extends Visitor<AstConstant> {
         compiler.reportError(
             node.condition, MessageKind.NOT_ASSIGNABLE,
             {'fromType': conditionType, 'toType': compiler.boolClass.rawType});
+        // TODO(ahe): Don't throw, recover from error.
+        throw new CompilerCancelledException(null);
       }
       return null;
     }