Parse 'x.y' as PrefixedIdentifier, not PropertyAccess with Fasta.

pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart

Index: pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
index 9947775894f4b28d423fb7153acb2d7a27d0db4b..bcf7a20a625e67d2f40d3c1ee229cd42c2ae8c72 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -254,8 +254,13 @@ class AstBuilder extends ScopeListener {
     Expression identifierOrInvoke = pop();
     Expression receiver = pop();
     if (identifierOrInvoke is SimpleIdentifier) {
-      push(ast.propertyAccess(
-          receiver, toAnalyzerToken(token), identifierOrInvoke));
+      if (receiver is SimpleIdentifier && identical('.', token.stringValue)) {
+        push(ast.prefixedIdentifier(
+            receiver, toAnalyzerToken(token), identifierOrInvoke));
+      } else {
+        push(ast.propertyAccess(
+            receiver, toAnalyzerToken(token), identifierOrInvoke));
+      }
     } else if (identifierOrInvoke is MethodInvocation) {
       assert(identifierOrInvoke.target == null);
       identifierOrInvoke