fix(chips): fix chip list focus and keyboard behaviors

Author: tinayuangao

src/lib/chips/chip-list.ts

@@ -432,9 +432,9 @@ export class MdChipList implements MdFormFieldControl<any>, ControlValueAccessor
 
     let isPrevKey = (code === (isRtl ? RIGHT_ARROW : LEFT_ARROW));
     let isNextKey = (code === (isRtl ? LEFT_ARROW : RIGHT_ARROW));
-    let isBackKey = (code === BACKSPACE || code == DELETE || code == UP_ARROW || isPrevKey);
+    let isDeleteKey = code === BACKSPACE || code == DELETE;
     // If they are on an empty input and hit backspace/delete/left arrow, focus the last chip
-    if (isInputEmpty && isBackKey) {
+    if (isInputEmpty && isDeleteKey) {
       this._keyManager.setLastItemActive();
       event.preventDefault();
       return;
@@ -504,8 +504,6 @@ export class MdChipList implements MdFormFieldControl<any>, ControlValueAccessor
       if (focusChip) {
         focusChip.focus();
       }
-    } else if (chipsArray.length === 0) {
-      this._focusInput();
     }
 
     // Reset our destroyed index