Fix Unicode issues in dart2js and dart2dart.

sdk/lib/_internal/compiler/implementation/string_validator.dart

Index: sdk/lib/_internal/compiler/implementation/string_validator.dart
diff --git a/sdk/lib/_internal/compiler/implementation/string_validator.dart b/sdk/lib/_internal/compiler/implementation/string_validator.dart
index a5f15e218067e6b85a39ba45c28ced379726c840..c48611dc9f52ac1afd00b9a1556bff96d05aebea 100644
--- a/sdk/lib/_internal/compiler/implementation/string_validator.dart
+++ b/sdk/lib/_internal/compiler/implementation/string_validator.dart
@@ -96,6 +96,8 @@ class StringValidator {
     int length = 0;
     int index = startOffset;
     bool containsEscape = false;
+    bool previousWasLeadSurrogate = false;
+    bool invalidUtf16 = false;
     for(Iterator<int> iter = string.iterator(); iter.hasNext; length++) {
       index++;
       int code = iter.next();
@@ -168,15 +170,27 @@ class StringValidator {
           code = value;
         }
       }
+      if (code >= 0x10000) length++;
+      assert(code <= 0x10ffff);
       // This handles both unescaped characters and the value of unicode
       // escapes.
-      if (!isUnicodeScalarValue(code)) {
-        stringParseError(
-            "Invalid Unicode scalar value U+${code.toRadixString(16)}",
-            token, index);
-        return null;
+      if (previousWasLeadSurrogate) {
+        if (!isUtf16TrailSurrogate(code)) {
+          invalidUtf16 = true;
+          break;
+        }
+        previousWasLeadSurrogate = false;
+      } else if (isUtf16LeadSurrogate(code)) {
+        previousWasLeadSurrogate = true;
+      } else if (!isUnicodeScalarValue(code)) {
+        invalidUtf16 = true;
+        break;
       }
     }
+    if (previousWasLeadSurrogate || invalidUtf16) {
+      stringParseError("Invalid Utf16 surrogate", token, index);
+      return null;
+    }
     // String literal successfully validated.
     if (quoting.raw || !containsEscape) {
       // A string without escapes could just as well have been raw.