Remove warnings when building the dart2js sdk.

tools/patch_sdk.dart

Index: tools/patch_sdk.dart
diff --git a/tools/patch_sdk.dart b/tools/patch_sdk.dart
index 81f239f3e29f06f156817f5f45b474181d21c54f..8e86120e9c5a7e37327dc2c5850a840f4c7adccf 100644
--- a/tools/patch_sdk.dart
+++ b/tools/patch_sdk.dart
@@ -597,8 +597,9 @@ class PatchApplier extends GeneralizingAstVisitor {
     var name = _qualifiedName(node);
     var patchNode = patch.patches[name];
     if (patchNode == null) {
-      if (externalKeyword != null) {
+      if (externalKeyword != null && _shouldHaveImplementation(name)) {
         print('warning: patch not found for $name: $node');
+        exitCode = 1;
       }
       return;
     }
@@ -615,6 +616,23 @@ class PatchApplier extends GeneralizingAstVisitor {
   }
 }
 
+/// Whether a member should have an implementation after patching the SDK.
+///
+/// True for most members except for the *.fromEnvironment constructors under
+/// the dart2js target.
+bool _shouldHaveImplementation(String qualifiedName) {
+  if (!forDart2js) return true;
+  // Note: dart2js implements int.fromEnvironment, bool.fromEnvironment
+  // and String.fromEnvironment directly and expects the SDK code to
+  // have an external declaration.
+  var isFromEnvironment = const [
+    'bool.fromEnvironment',
+    'int.fromEnvironment',
+    'String.fromEnvironment'
+  ].contains(qualifiedName);
+  return !isFromEnvironment;
+}
+
 class PatchFinder extends GeneralizingAstVisitor {
   final String contents;
   final CompilationUnit unit;