Convert String to a class.

lib/compiler/implementation/lib/core_patch.dart

Index: lib/compiler/implementation/lib/core_patch.dart
diff --git a/lib/compiler/implementation/lib/core_patch.dart b/lib/compiler/implementation/lib/core_patch.dart
index f47c27b840561e48ef14845cb5d63c9289a49fa5..a4844096bb9ae5b01f92ec2cca66b35259697b3e 100644
--- a/lib/compiler/implementation/lib/core_patch.dart
+++ b/lib/compiler/implementation/lib/core_patch.dart
@@ -83,3 +83,54 @@ patch class NoSuchMethodError {
     return Primitives.objectToString(object);
   }
 }
+
+
+// Patch for String implementation.
+// TODO(ager): Split out into date_patch.dart and allow #source
+// in patch files?
+patch class String {
+  patch factory String.fromCharCodes(List<int> charCodes) {
+    checkNull(charCodes);
+    if (!isJsArray(charCodes)) {
+      if (charCodes is !List) throw new IllegalArgumentException(charCodes);
+      charCodes = new List.from(charCodes);
+    }
+    return Primitives.stringFromCharCodes(charCodes);
+  }
+}
+
+patch class Strings {
+  patch static String join(List<String> strings, String separator) {
+    checkNull(strings);
+    checkNull(separator);
+    if (separator is !String) throw new IllegalArgumentException(separator);
+    return stringJoinUnchecked(_toJsStringArray(strings), separator);
+  }
+
+  patch static String concatAll(List<String> strings) {
+    return stringJoinUnchecked(_toJsStringArray(strings), "");
+  }
+
+  static List _toJsStringArray(List<String> strings) {
+    checkNull(strings);
+    var array;
+    final length = strings.length;
+    if (isJsArray(strings)) {
+      array = strings;
+      for (int i = 0; i < length; i++) {
+        final string = strings[i];
+        checkNull(string);
+        if (string is !String) throw new IllegalArgumentException(string);
+      }
+    } else {
+      array = new List(length);
+      for (int i = 0; i < length; i++) {
+        final string = strings[i];
+        checkNull(string);
+        if (string is !String) throw new IllegalArgumentException(string);
+        array[i] = string;
+      }
+    }
+    return array;
+  }
+}