Fix parseAssignableSelector() tests and parse '?.' 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 5a14cae0ff85418224322eeffdf0f131ad1e142a..e4f520f65a537175e1216a83c631ba4b93cd752f 100644
--- a/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
+++ b/pkg/front_end/lib/src/fasta/analyzer/ast_builder.dart
@@ -238,6 +238,7 @@ class AstBuilder extends ScopeListener {
   void handleBinaryExpression(Token token) {
     debugEvent("BinaryExpression");
     if (identical(".", token.stringValue) ||
+        identical("?.", token.stringValue) ||
         identical("..", token.stringValue)) {
       doDotExpression(token);
     } else {
@@ -1174,7 +1175,8 @@ class AstBuilder extends ScopeListener {
     // TODO(paulberry): capture doc comments.  See dartbug.com/28851.
     Comment comment = null;
     Token period;
-    void unnamedConstructor(SimpleIdentifier returnType, SimpleIdentifier name) {
+    void unnamedConstructor(
+        SimpleIdentifier returnType, SimpleIdentifier name) {
       push(ast.constructorDeclaration(
           comment,
           metadata,