Make mappedBy lazy.

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

Index: sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
index ddea8fc00a2cc6c142a72eb3931457da9a612027..1a7de95758c8a4d58c31fde24534003a8565f58e 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -216,38 +216,30 @@ patch class String {
 
 // Patch for String implementation.
 patch class Strings {
-  patch static String join(List<String> strings, String separator) {
+  patch static String join(Iterable<String> strings, String separator) {
     checkNull(strings);
     checkNull(separator);
     if (separator is !String) throw new ArgumentError(separator);
     return stringJoinUnchecked(_toJsStringArray(strings), separator);
   }
 
-  patch static String concatAll(List<String> strings) {
+  patch static String concatAll(Iterable<String> strings) {
     return stringJoinUnchecked(_toJsStringArray(strings), "");
   }
 
-  static List _toJsStringArray(List<String> strings) {
+  static List _toJsStringArray(Iterable<String> strings) {
     checkNull(strings);
     var array;
+    if (!isJsArray(strings)) {
+      strings = new List.from(string);
+    }
     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 ArgumentError(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 ArgumentError(string);
-        array[i] = string;
-      }
+    for (int i = 0; i < length; i++) {
+      final string = strings[i];
+      checkNull(string);
+      if (string is !String) throw new ArgumentError(string);
     }
-    return array;
+    return strings;
   }
 }