fix(input): fix placeholder for number input with bad input. (#2362)

* fix(input): fix placeholder for number input with bad input.

* addressed comments

* add comment

Author: mmalerba

src/lib/input/input-container.ts

@@ -152,7 +152,14 @@ export class MdInputDirective {
    */
   @Output() _placeholderChange = new EventEmitter<string>();
 
-  get empty() { return (this.value == null || this.value === '') && !this._isNeverEmpty(); }
+  get empty() {
+    return !this._isNeverEmpty() &&
+        (this.value == null || this.value === '') &&
+        // Check if the input contains bad input. If so, we know that it only appears empty because
+        // the value failed to parse. From the user's perspective it is not empty.
+        // TODO(mmalerba): Add e2e test for bad input case.
+        !this._isBadInput();
+  }
 
   private get _uid() { return this._cachedUid = this._cachedUid || `md-input-${nextUniqueId++}`; }
 
@@ -199,6 +206,10 @@ export class MdInputDirective {
 
   private _isNeverEmpty() { return this._neverEmptyInputTypes.indexOf(this._type) !== -1; }
 
+  private _isBadInput() {
+    return (this._elementRef.nativeElement as HTMLInputElement).validity.badInput;
+  }
+
   /** Determines if the component host is a textarea. If not recognizable it returns false. */
   private _isTextarea() {
     let nativeElement = this._elementRef.nativeElement;