Introduce CompletionTarget to take the place of the Node being completed.

pkg/analyzer/lib/src/generated/element_resolver.dart

Index: pkg/analyzer/lib/src/generated/element_resolver.dart
diff --git a/pkg/analyzer/lib/src/generated/element_resolver.dart b/pkg/analyzer/lib/src/generated/element_resolver.dart
index a698d182db9aa99d116ec3a731378f1b67b1c97f..2b55cfbb9eb3c2f88bddc0e489610ab74d20e488 100644
--- a/pkg/analyzer/lib/src/generated/element_resolver.dart
+++ b/pkg/analyzer/lib/src/generated/element_resolver.dart
@@ -3100,6 +3100,14 @@ class SyntheticIdentifier extends Identifier {
   Element get bestElement => null;
 
   @override
+  Iterable get childEntities {
+    // Should never be called, since a SyntheticIdentifier never appears in the
+    // AST--it is just used for lookup.
+    assert(false);
+    return new ChildEntities();
+  }
+
+  @override
   sc.Token get endToken => null;
 
   @override