fix(cloudwatch): AlarmWidget takes IAlarm (#3219)

This allows using an imported alarm.

Fixes #3068.

Author: rix0rrr

packages/@aws-cdk/aws-cloudwatch/lib/graph.ts

@@ -1,5 +1,5 @@
 import cdk = require('@aws-cdk/core');
-import { Alarm } from "./alarm";
+import { IAlarm } from "./alarm";
 import { IMetric } from "./metric-types";
 import { ConcreteWidget } from "./widget";
 
@@ -74,7 +74,7 @@ export interface AlarmWidgetProps extends MetricWidgetProps {
   /**
    * The alarm to show
    */
-  readonly alarm: Alarm;
+  readonly alarm: IAlarm;
 
   /**
    * Left Y axis

packages/@aws-cdk/aws-cloudwatch/test/test.graphs.ts

@@ -1,6 +1,6 @@
 import { Stack } from '@aws-cdk/core';
 import { Test } from 'nodeunit';
-import { AlarmWidget, GraphWidget, Metric, Shading, SingleValueWidget } from '../lib';
+import { Alarm, AlarmWidget, GraphWidget, Metric, Shading, SingleValueWidget } from '../lib';
 
 export = {
   'add metrics to graphs on either axis'(test: Test) {
@@ -250,4 +250,20 @@ export = {
 
     test.done();
   },
+
+  'can use imported alarm with graph'(test: Test) {
+    // GIVEN
+    const stack = new Stack();
+    const alarm = Alarm.fromAlarmArn(stack, 'Alarm', 'arn:aws:cloudwatch:region:account-id:alarm:alarm-name');
+
+    // WHEN
+    new AlarmWidget({
+      title: 'My fancy graph',
+      alarm
+    });
+
+    // THEN: Compiles
+
+    test.done();
+  },
 };