Don't attempt to recover from missing strings.

pkg/compiler/lib/src/parser/element_listener.dart

Index: pkg/compiler/lib/src/parser/element_listener.dart
diff --git a/pkg/compiler/lib/src/parser/element_listener.dart b/pkg/compiler/lib/src/parser/element_listener.dart
index 10fd6980d01f91b6627acbcadc00f64121ced301..11d710850dc3b9f331385a7ccfb1aff63fa668fa 100644
--- a/pkg/compiler/lib/src/parser/element_listener.dart
+++ b/pkg/compiler/lib/src/parser/element_listener.dart
@@ -612,8 +612,10 @@ class ElementListener extends Listener {
         break;
 
       case ErrorKind.ExpectedString:
-        errorCode = MessageKind.STRING_EXPECTED;
-        break;
+        reportFatalError(
+            reporter.spanFromToken(token),
+            "Expected a String, but got '${token.value}'.");
+        return null;
 
       case ErrorKind.ExtraneousModifier:
         errorCode = MessageKind.EXTRANEOUS_MODIFIER;