[es7] implement |do| expressions proposal

src/preparser.cc

Index: src/preparser.cc
diff --git a/src/preparser.cc b/src/preparser.cc
index ecf4a950c0ea8e76192e205020de4713bafb5f40..74e11b1d454b3b33a3f6bcadb18f91476a246f37 100644
--- a/src/preparser.cc
+++ b/src/preparser.cc
@@ -1239,6 +1239,23 @@ PreParser::Expression PreParser::ParseV8Intrinsic(bool* ok) {
   return Expression::Default();
 }
 
+
+PreParserExpression PreParser::ParseDoExpression(bool* ok) {
+  // AssignmentExpression ::
+  //     do '{' StatementList '}'
+  Expect(Token::DO, CHECK_OK);
+  Expect(Token::LBRACE, CHECK_OK);
+  Scope* block_scope = NewScope(scope_, BLOCK_SCOPE);
+  {
+    BlockState block_state(&scope_, block_scope);
+    while (peek() != Token::RBRACE) {
+      ParseStatementListItem(CHECK_OK);
+    }
+    Expect(Token::RBRACE, CHECK_OK);
+    return PreParserExpression::Default();
+  }
+}
+
 #undef CHECK_OK