Improve parser error recovery.

dart/sdk/lib/_internal/compiler/implementation/compiler.dart

Index: dart/sdk/lib/_internal/compiler/implementation/compiler.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
index 7b8420a4f51f4758d2cd94d9680ea989ad89bbff..e0b01b2b8b04882bb2994123d4d6eb7e21118a0c 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/compiler.dart
@@ -1264,7 +1264,7 @@ abstract class Compiler implements DiagnosticListener {
                        "library."});
         }
       } else {
-        if (main.isErroneous) {
+        if (main.isErroneous && main.isSynthesized) {
           reportFatalError(main, MessageKind.GENERIC,
               {'text': "Cannot determine which '$MAIN' to use."});
         } else if (!main.isFunction) {
@@ -1652,10 +1652,11 @@ abstract class Compiler implements DiagnosticListener {
   }
 
   SourceSpan spanFromElement(Element element) {
-    if (Elements.isErroneousElement(element)) {
+    while (element != null && element.isSynthesized) {
       element = element.enclosingElement;
     }
-    if (element.position == null &&
+    if (element != null &&
+        element.position == null &&
         !element.isLibrary &&
         !element.isCompilationUnit) {
       // Sometimes, the backend fakes up elements that have no