add 'get' method to common observable map api

src/observable/map/ApplyMap.js

@@ -82,4 +82,8 @@ export class ApplyMap extends BaseObservableMap {
     get size() {
         return this._source.size;
     }
+
+    get(key) {
+        return this._source.get(key);
+    }
 }

src/observable/map/BaseObservableMap.js

@@ -49,4 +49,9 @@ export class BaseObservableMap extends BaseObservable {
     get size() {
         throw new Error("unimplemented");
     }
+
+    // eslint-disable-next-line no-unused-vars
+    get(key) {
+        throw new Error("unimplemented");
+    }
 }

src/observable/map/FilteredMap.js

@@ -128,6 +128,13 @@ export class FilteredMap extends BaseObservableMap {
         });
         return count;
     }
+
+    get(key) {
+        const value = this._source.get(key);
+        if (value && this._filter(value, key)) {
+            return value;
+        }
+    }
 }
 
 class FilterIterator {