Implement external strings.

runtime/lib/string.dart

Index: runtime/lib/string.dart
diff --git a/runtime/lib/string.dart b/runtime/lib/string.dart
index 318b01c1358b946d33bb32a78c7b43e102b52838..b4fbc431f8fd1f03cc302fb2168eb201608e723c 100644
--- a/runtime/lib/string.dart
+++ b/runtime/lib/string.dart
@@ -413,6 +413,43 @@ class FourByteString extends StringBase implements String {
   }
 }
 
+
+class ExternalOneByteString extends StringBase implements String {
+  // Checks for one-byte whitespaces only.
+  // TODO(srdjan): Investigate if 0x85 (NEL) and 0xA0 (NBSP) are valid
+  // whitespaces for one byte strings.
+  bool _isWhitespace(int codePoint) {
+    return
+      (codePoint === 32) || // Space.
+      ((9 <= codePoint) && (codePoint <= 13)); // CR, LF, TAB, etc.
+  }
+}
+
+
+class ExternalTwoByteString extends StringBase implements String {
+  // Checks for one-byte whitespaces only.
+  // TODO(srdjan): Investigate if 0x85 (NEL) and 0xA0 (NBSP) are valid
+  // whitespaces. Add checking for multi-byte whitespace codepoints.
+  bool _isWhitespace(int codePoint) {
+    return
+      (codePoint === 32) || // Space.
+      ((9 <= codePoint) && (codePoint <= 13)); // CR, LF, TAB, etc.
+  }
+}
+
+
+class ExternalFourByteString extends StringBase implements String {
+  // Checks for one-byte whitespaces only.
+  // TODO(srdjan): Investigate if 0x85 (NEL) and 0xA0 (NBSP) are valid
+  // whitespaces. Add checking for multi-byte whitespace codepoints.
+  bool _isWhitespace(int codePoint) {
+    return
+      (codePoint === 32) || // Space.
+      ((9 <= codePoint) && (codePoint <= 13)); // CR, LF, TAB, etc.
+  }
+}
+
+
 class _StringMatch implements Match {
   const _StringMatch(int this._start,
                      String this.str,