Preserve the contents of Dart strings with unmatched surrogate halfs by avoiding a UTF16 -> UTF8

runtime/bin/vmservice/observatory/test/string_escaping_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library string_escaping_test;
 
 import 'dart:async';
 import 'dart:mirrors';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
 var ascii;
 var latin1;
 var unicode;
 var hebrew;
 var singleQuotes;
 var doubleQuotes;
 var newLines;
 var tabs;
 var suggrogatePairs;
 var nullInTheMiddle;
 var escapedUnicodeEscape;
 var longStringEven;
 var longStringOdd;
+var malformedWithLeadSurrogate;
+var malformedWithTrailSurrogate;
 
 void script() {
   ascii = "Hello, World!";
   latin1 = "blåbærgrød";
   unicode = "Îñţérñåţîöñåļîžåţîờñ";
   hebrew = "שלום רב שובך צפורה נחמדת";  // Right-to-left text.
   singleQuotes = "'One,' he said.";
   doubleQuotes = '"Two," he said.';
   newLines = "Windows\r\nSmalltalk\rUnix\n";
   tabs = "One\tTwo\tThree";
   suggrogatePairs = "1𝄞2𝄞𝄞3𝄞𝄞𝄞";
   nullInTheMiddle = "There are four\u0000 words.";
   escapedUnicodeEscape = "Should not be A: \\u0041";
 
   // A surrogate pair will cross the preferred truncation boundry.
   longStringEven = "..";
   for (int i = 0; i < 512; i++) longStringEven += "𝄞";
   longStringOdd = ".";
   for (int i = 0; i < 512; i++) longStringOdd += "𝄞";
+
+  malformedWithLeadSurrogate = "before" + "𝄞"[0] + "after";
+  malformedWithTrailSurrogate = "before" + "𝄞"[1] + "after";
 }
 
 var tests = [
 
 (Isolate isolate) =>
   isolate.rootLib.load().then((Library lib) {
     expectFullString(String varName, String varValueAsString) {
       ServiceMap field = lib.variables.singleWhere((v) => v.name == varName);
       Instance value = field['value'];
       expect(value.valueAsString, equals(varValueAsString));
       expect(value.valueAsStringIsTruncated, isFalse);
     }
     expectTruncatedString(String varName, String varValueAsString) {
       ServiceMap field = lib.variables.singleWhere((v) => v.name == varName);
       Instance value = field['value'];
       expect(varValueAsString, startsWith(value.valueAsString));
       expect(value.valueAsStringIsTruncated, isTrue);
     }
 
     script();  // Need to initialize variables in the testing isolate.
     expectFullString('ascii', ascii);
     expectFullString('latin1', latin1);
     expectFullString('unicode', unicode);
     expectFullString('hebrew', hebrew);
     expectFullString('singleQuotes', singleQuotes);
     expectFullString('doubleQuotes', doubleQuotes);
     expectFullString('newLines', newLines);
     expectFullString('tabs', tabs);
     expectFullString('suggrogatePairs', suggrogatePairs);
-    expectFullString('nullInTheMiddle', nullInTheMiddle);  /// 01: ok
+    expectFullString('nullInTheMiddle', nullInTheMiddle);
     expectTruncatedString('longStringEven', longStringEven);
     expectTruncatedString('longStringOdd', longStringOdd);
+    expectFullString('malformedWithLeadSurrogate', malformedWithLeadSurrogate);
+    expectFullString('malformedWithTrailSurrogate', malformedWithTrailSurrogate);
   }),
 
 ];
 
 main(args) => runIsolateTests(args, tests, testeeBefore: script);