implement .size for all observable maps

as SortedMapList uses it, putting undefined in its list
initially when missing, creating a crash in
the TemplateView that renders it

src/observable/map/ApplyMap.js

@@ -78,4 +78,8 @@ export class ApplyMap extends BaseObservableMap {
     [Symbol.iterator]() {
         return this._source[Symbol.iterator]();
     }
+
+    get size() {
+        return this._source.size;
+    }
 }

src/observable/map/BaseObservableMap.js

@@ -41,4 +41,12 @@ export class BaseObservableMap extends BaseObservable {
             h.onRemove(key, value);
         }
     }
+
+    [Symbol.iterator]() {
+        throw new Error("unimplemented");
+    }
+
+    get size() {
+        throw new Error("unimplemented");
+    }
 }

src/observable/map/FilteredMap.js

@@ -71,10 +71,11 @@ export class FilteredMap extends BaseObservableMap {
     }
 
     onRemove(key, value) {
-        if (this._filter && !this._included.get(key)) {
-            return;
+        const wasIncluded = !this._filter || this._included.get(key);
+        this._included.delete(key);
+        if (wasIncluded) {
+            this.emitRemove(key, value);
         }
-        this.emitRemove(key, value);
     }
 
     onUpdate(key, value, params) {
@@ -117,6 +118,16 @@ export class FilteredMap extends BaseObservableMap {
     [Symbol.iterator]() {
         return new FilterIterator(this._source, this._included);
     }
+
+    get size() {
+        let count = 0;
+        this._included.forEach(included => {
+            if (included) {
+                count += 1;
+            }
+        });
+        return count;
+    }
 }
 
 class FilterIterator {