Fix assertion not holding (calling with start==end) and typo.

runtime/lib/convert_patch.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.
 
 import "dart:_internal" show POWERS_OF_TEN;
 
 // JSON conversion.
 
 patch _parseJson(String json, reviver(var key, var value)) {
   _BuildJsonListener listener;
@@ skipping 927 matching lines @@
     int start = position;
     int end = chunkEnd;
     while (position < end) {
       int char = getChar(position++);
       // BACKSLASH is larger than QUOTE and SPACE.
       if (char > BACKSLASH) {
         continue;
       }
       if (char == BACKSLASH) {
         beginString();
-        addSliceToString(start, position - 1);
+        int sliceEnd = position - 1;
+        if (start < sliceEnd) addSliceToString(start, sliceEnd);
         return parseStringToBuffer(position - 1);
       }
       if (char == QUOTE) {
         listener.handleString(getString(start, position - 1));
         return position;
       }
       if (char < SPACE) {
         fail(position - 1, "Control character in string");
       }
     }
     beginString();
-    addSliceToString(start, end);
+    if (start < end) addSliceToString(start, end);
     return chunkString(STR_PLAIN);
   }
 
   /**
    * Sets up a partial string state.
    *
    * The state is either not inside an escape, or right after a backslash.
    * For partial strings ending inside a Unicode escape, use
    * [chunkStringEscapeU].
    */
@@ skipping 657 matching lines @@
   List<int> chunk;
   int chunkEnd;
 
   _JsonUtf8Parser(_JsonListener listener, this.allowMalformed)
       : super(listener);
 
   int getChar(int position) => chunk[position];
 
   String getString(int start, int end) {
     beginString();
-    addSliceToString(start, end);
+    if (start < end) addSliceToString(start, end);
     String result = endString();
     return result;
   }
 
   void beginString() {
     this.buffer = new _Utf8StringBuffer(allowMalformed);
   }
 
   void addSliceToString(int start, int end) {
     _Utf8StringBuffer buffer = this.buffer;
@@ skipping 61 matching lines @@
     _parser.parse(start);
   }
 
   void close() {
     _parser.close();
     var decoded = _parser.result;
     _sink.add(decoded);
     _sink.close();
   }
 }