Support strictNullChecks and resetting the control

Author: ppham27

src/demo-app/select/select-demo.ts

@@ -16,7 +16,7 @@ export class SelectDemo {
   pokemonDisabled = false;
   showSelect = false;
   currentDrink: string;
-  currentDrinkObject: {} = {value: 'tea-5', viewValue: 'Tea'};
+  currentDrinkObject: {}|undefined = {value: 'tea-5', viewValue: 'Tea'};
   currentPokemon: string[];
   currentPokemonFromGroup: string;
   latestChangeEvent: MdSelectChange;
@@ -106,11 +106,7 @@ export class SelectDemo {
   }
 
   setDrinkObjectByCopy(selectedDrink: {}) {
-    if (selectedDrink) {
-      this.currentDrinkObject = Object.assign({}, selectedDrink);
-    } else {
-      this.currentDrinkObject = undefined;
-    }
+    this.currentDrinkObject = selectedDrink ? {...selectedDrink} : undefined;
   }
 
   compareDrinkObjectsByValue(d1: {value: string}, d2: {value: string}) {

src/lib/select/select.ts

@@ -623,6 +623,10 @@ export class MdSelect extends _MdSelectMixinBase implements AfterContentInit, On
     const optionsArray = this.options.toArray();
 
     const correspondingOption = optionsArray.find((option: MdOption) => {
+      // Treat null as a special reset value.
+      if (option.value === null) {
+        return false;
+      }
       try {
         return this._compareWith(option.value,  value);
       } catch (error) {