Don't crash when generating error messages with malformed strings.

runtime/vm/unicode.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/unicode.h"
 
 #include "vm/allocation.h"
 #include "vm/globals.h"
 #include "vm/object.h"
 
@@ skipping 120 matching lines @@
   String::CodePointIterator it(str);
   while (it.Next()) {
     int32_t ch = it.Current();
     length += Utf8::Length(ch);
   }
   return length;
 }
 
 
 intptr_t Utf8::Encode(int32_t ch, char* dst) {
-  ASSERT(!Utf16::IsSurrogate(ch));
   static const int kMask = ~(1 << 6);
   if (ch <= kMaxOneByteChar) {
     dst[0] = ch;
     return 1;
   }
   if (ch <= kMaxTwoByteChar) {
     dst[0] = 0xC0 | (ch >> 6);
     dst[1] = 0x80 | (ch & kMask);
     return 2;
   }
@@ skipping 144 matching lines @@
 
 
 void Utf16::Encode(int32_t codepoint, uint16_t* dst) {
   ASSERT(codepoint > Utf16::kMaxCodeUnit);
   ASSERT(dst != NULL);
   dst[0] = (Utf16::kLeadSurrogateOffset + (codepoint >> 10));
   dst[1] = (0xDC00 + (codepoint & 0x3FF));
 }
 
 }  // namespace dart