Make mappedBy lazy.

runtime/lib/string_base.dart

Index: runtime/lib/string_base.dart
diff --git a/runtime/lib/string_base.dart b/runtime/lib/string_base.dart
index eef5b02a47d363fdcf73089036c762d77ce49f81..b167e8027e5395da6244fca887100741092cfa81 100644
--- a/runtime/lib/string_base.dart
+++ b/runtime/lib/string_base.dart
@@ -332,34 +332,36 @@ class _StringBase {
   String toLowerCase() native "String_toLowerCase";
 
   // Implementations of Strings methods follow below.
-  static String join(List<String> strings, String separator) {
-    final int length = strings.length;
-    if (length == 0) {
-      return "";
-    }
-
-    List stringsList = strings;
-    if (separator.length != 0) {
-      stringsList = new List(2 * length - 1);
-      stringsList[0] = strings[0];
-      int j = 1;
-      for (int i = 1; i < length; i++) {
-        stringsList[j++] = separator;
-        stringsList[j++] = strings[i];
+  static String join(Iterable<String> strings, String separator) {
+    bool first = true;
+    List<String> stringsList = <String>[];
+    for (String string in strings) {
+      if (first) {
+        first = false;
+      } else {
+        stringsList.add(separator);
       }
+
+      if (string is! String) throw new ArgumentError(string);
+      stringsList.add(string);
     }
     return concatAll(stringsList);
   }
 
-  static String concatAll(List<String> strings) {
+  static String concatAll(Iterable<String> strings) {
     _ObjectArray stringsArray;
     if (strings is _ObjectArray) {
       stringsArray = strings;
+      for (int i = 0; i < strings.length; i++) {
+        if (strings[i] is! String) throw new ArgumentError(strings[i]);
+      }
     } else {
       int len = strings.length;
       stringsArray = new _ObjectArray(len);
-      for (int i = 0; i < len; i++) {
-        stringsArray[i] = strings[i];
+      int i = 0;
+      for (String string in strings) {
+        if (string is! String) throw new ArgumentError(string);
+        stringsArray[i++] = string;
       }
     }
     return _concatAll(stringsArray);