Revert "Allow individual unicode surrogates 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 84 matching lines @@
           is_malformed |= !IsTrailByte(code_unit);
           ch = (ch << 6) + code_unit;
         } else {
           return false;
         }
       }
       ch -= kMagicBits[num_trail_bytes];
       if (!((is_malformed == false) &&
             (j == num_trail_bytes) &&
             !Utf::IsOutOfRange(ch) &&
-            !IsNonShortestForm(ch, j))) {
+            !IsNonShortestForm(ch, j) &&
+            !Utf16::IsSurrogate(ch))) {
         return false;
       }
     }
     i += j;
   }
   return true;
 }
 
 
 intptr_t Utf8::Length(int32_t ch) {
@@ skipping 77 matching lines @@
         ch = (ch << 6) + code_unit;
       } else {
         *dst = -1;
         return 0;
       }
     }
     ch -= kMagicBits[num_trail_bytes];
     if (!((is_malformed == false) &&
           (i == num_trail_bytes) &&
           !Utf::IsOutOfRange(ch) &&
-          !IsNonShortestForm(ch, i))) {
+          !IsNonShortestForm(ch, i) &&
+          !Utf16::IsSurrogate(ch))) {
       *dst = -1;
       return 0;
     }
   }
   *dst = ch;
   return i;
 }
 
 
 bool Utf8::DecodeToLatin1(const uint8_t* utf8_array,
@@ skipping 79 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