fix(input): don't dirty model when input event triggered due to placeholder change

Certain versions of IE inexplicably trigger an input event in response to a placeholder
being set.

It is not possible to sniff for this behaviour nicely as the event is not triggered if
the element is not attached to the document, and the event triggers asynchronously so
it is not possible to accomplish this without deferring DOM compilation and slowing down
load times.

Closes angular#2614
Closes angular#5960

Author: caitp

src/ng/directive/input.js

@@ -405,6 +405,8 @@ function validate(ctrl, validatorName, validity, value){
 }
 
 function textInputType(scope, element, attr, ctrl, $sniffer, $browser) {
+  var lastValue = element.val();
+  var lastPlaceholder = element[0].placeholder;
   // In composition mode, users are still inputing intermediate text buffer,
   // hold the listener until composition is done.
   // More about composition events: https://developer.mozilla.org/en-US/docs/Web/API/CompositionEvent
@@ -420,17 +422,24 @@ function textInputType(scope, element, attr, ctrl, $sniffer, $browser) {
     });
   }
 
-  var listener = function() {
+  var listener = function(event) {
     if (composing) return;
     var value = element.val();
 
+    if (msie && event && event.type === 'input' && !value && value === lastValue &&
+      element[0].placeholder !== lastPlaceholder) {
+      lastPlaceholder = element[0].placeholder;
+      return;
+    }
+
     // By default we will trim the value
     // If the attribute ng-trim exists we will avoid trimming
     // e.g. <input ng-model="foo" ng-trim="false">
     if (toBoolean(attr.ngTrim || 'T')) {
       value = trim(value);
     }
 
+    lastValue = value;
     if (ctrl.$viewValue !== value) {
       if (scope.$$phase) {
         ctrl.$setViewValue(value);

test/ng/directive/inputSpec.js

@@ -509,6 +509,43 @@ describe('input', function() {
     });
   }
 
+
+  it('should not dirty the model on an input event in response to a placeholder change', inject(function($document) {
+    var $body = $document.find('body');
+    compileInput('<input type="text" ng-model="name" name="name" placeholder="{{placeholder}}" />');
+    $body.append(formElm);
+    var waited = false;
+
+    scope.$apply(function() {
+      scope.placeholder = "Test";
+    });
+    setTimeout(function() { waited = true; }, 500);
+
+    waitsFor(function() {
+      return waited;
+    }, 1000);
+
+    runs(function() {
+      expect(inputElm.attr('placeholder')).toBe('Test');
+      expect(inputElm).toBePristine();
+      waited = false;
+      scope.$apply(function() {
+        scope.placeholder = "Test Again";
+      });
+      setTimeout(function() { waited = true; }, 500);
+
+      waitsFor(function() {
+        return waited;
+      }, 1000);
+      runs(function() {
+        expect(inputElm.attr('placeholder')).toBe('Test Again');
+        expect(inputElm).toBePristine();
+        inputElm.remove();
+      });
+    });
+  }));
+
+
   describe('"change" event', function() {
     function assertBrowserSupportsChangeEvent(inputEventSupported) {
       // Force browser to report a lack of an 'input' event