Add a couple of tests.

test/src/coreutils/observablemap.spec.ts

@@ -20,6 +20,14 @@ describe('@jupyterlab/coreutils', () => {
       });
     });
 
+    describe('#type', () => {
+
+      it('should return `Map`', () => {
+        let value = new ObservableMap<number>();
+        expect(value.type).to.be('Map');
+      });
+    });
+
     describe('#size', ()=>{
       it('should return the number of entries in the map', ()=>{
         let value = new ObservableMap<number>();

test/src/coreutils/observablestring.spec.ts

@@ -31,6 +31,14 @@ describe('@jupyterlab/coreutils', () => {
 
     });
 
+    describe('#type', () => {
+
+      it('should return `String`', () => {
+        let value = new ObservableString();
+        expect(value.type).to.be('String');
+      });
+    });
+
     describe('#changed', () => {
 
       it('should be emitted when the string changes', () => {

test/src/coreutils/observablevector.spec.ts

@@ -31,6 +31,15 @@ describe('@jupyterlab/coreutils', () => {
 
     });
 
+    describe('#type', () => {
+
+      it('should return `Vector`', () => {
+        let value = new ObservableVector<number>();
+        expect(value.type).to.be('Vector');
+      });
+    });
+
+
     describe('#changed', () => {
 
       it('should be emitted when the vector changes state', () => {