Compute better names for source maps.

pkg/compiler/lib/src/elements/elements.dart

Index: pkg/compiler/lib/src/elements/elements.dart
diff --git a/pkg/compiler/lib/src/elements/elements.dart b/pkg/compiler/lib/src/elements/elements.dart
index dc8650408d5eda22cf29027819a3b56d05381456..4231737a7cd74671e52f4381b94e6dca9d000aae 100644
--- a/pkg/compiler/lib/src/elements/elements.dart
+++ b/pkg/compiler/lib/src/elements/elements.dart
@@ -992,7 +992,8 @@ abstract class MemberElement extends Element implements ExecutableElement {
 }
 
 /// A function, variable or parameter defined in an executable context.
-abstract class LocalElement extends Element implements TypedElement, Local {
+abstract class LocalElement extends Element
+    implements AstElement, TypedElement, Local {
 }
 
 /// A top level, static or instance field, a formal parameter or local variable.