Improve error messages.

pkg/front_end/lib/src/fasta/kernel/kernel_target.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
index 798e61b7ce04a51a4981d5b0588d6af8e029b6d5..78129b6b13e501a46dfadfad41091b0ea727bf7d 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_target.dart
@@ -109,6 +109,10 @@ class KernelTarget extends TargetImplementation {
     loader = createLoader();
   }
 
+  bool get hasErrors {
+    return errors.isNotEmpty || loader.collectCompileTimeErrors().isNotEmpty;
+  }
+
   void addError(file, int charOffset, String message) {
     Uri uri = file is String ? Uri.parse(file) : file;
     InputError error = new InputError(uri, charOffset, message);
@@ -281,7 +285,9 @@ class KernelTarget extends TargetImplementation {
       loader.finishStaticInvocations();
       finishAllConstructors();
       loader.finishNativeMethods();
-      runBuildTransformations();
+      if (!hasErrors) {
+        runBuildTransformations();

[ahe, 2017/06/09 08:13:14]

The parser reports a recoverable error on code like this:

class C {
  foo() async;
}

Fasta treats this as an abstract method and doesn't discard the async modifier.
The async transformer isn't prepared for that, so it crashes when it tries to
visit the null function body of C.foo above.

Before we would throw an InputError and never get as far as running
transformations.

+      }
 
       if (verify) this.verify();
       errors.addAll(loader.collectCompileTimeErrors().map((e) => e.format()));