Revert "Revert "Don't use Error.toSafeString in List constructor.""

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 372c4d41ead2cafc216b6aebe20836bff8a2593e..4b74f75622a54fe62927977df1a7fcc30bbf9381 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/core_patch.dart
@@ -162,15 +162,17 @@ patch class _StopwatchImpl {
 // Patch for List implementation.
 patch class List<E> {
   patch factory List([int length = 0]) {
+    // Explicit type test is necessary to protect Primitives.newGrowableList in
+    // unchecked mode.
     if ((length is !int) || (length < 0)) {
-      String lengthString = Error.safeToString(length);
-      throw new ArgumentError(
-          "Length must be a positive integer: $lengthString.");
+      throw new ArgumentError("Length must be a positive integer: $length.");
     }
     return Primitives.newGrowableList(length);
   }
 
   patch factory List.fixedLength(int length, {E fill: null}) {
+    // Explicit type test is necessary to protect Primitives.newFixedList in
+    // unchecked mode.
     if ((length is !int) || (length < 0)) {
       throw new ArgumentError("Length must be a positive integer: $length.");
     }
@@ -188,6 +190,8 @@ patch class List<E> {
    * filled with [fill].
    */
   patch factory List.filled(int length, E fill) {
+    // Explicit type test is necessary to protect Primitives.newGrowableList in
+    // unchecked mode.
     if ((length is !int) || (length < 0)) {
       throw new ArgumentError("Length must be a positive integer: $length.");
     }