diff --git a/src/vs/base/test/common/collections.test.ts b/src/vs/base/test/common/collections.test.ts
index 8db0058c7cdc1d2b1847427982f3dded09390594..1b9d41b109660889289e6231f1847d93a9e98037 100644
--- a/src/vs/base/test/common/collections.test.ts
+++ b/src/vs/base/test/common/collections.test.ts
@@ -31,6 +31,69 @@ suite('Collections', () => {
 		dict['toString'] = 123;
 		collections.forEach(dict, () => count++);
 		assert.equal(count, 1);
+
+		collections.forEach(dict, () => false);
+
+		collections.forEach(dict, (x, remove) => remove());
+		assert.equal(dict['toString'], null);
+
+		// don't iterate over properties that are not on the object itself
+		let test = Object.create({ 'derived': true });
+		collections.forEach(test, () => assert(false));
+	});
+
+	test('lookupOrInsert - should not insert if found', () => {
+		const property = 123;
+
+		let from = collections.createNumberDictionary();
+		from[property] = 'whatever';
+
+		collections.lookupOrInsert(from, property, () => assert(false));
+	});
+
+	test('lookupOrInsert - should insert if not found', () => {
+
+		const expected = 'alternate', property = 'test';
+
+		let fromWithValue = collections.createStringDictionary();
+		collections.lookupOrInsert(fromWithValue, property, expected);
+		assert.equal(fromWithValue[property], expected);
+
+		let fromWithCallback = collections.createStringDictionary();
+		collections.lookupOrInsert(fromWithCallback, property, () => expected);
+		assert.equal(fromWithCallback[property], expected);
+	});
+
+	test('groupBy', () => {
+
+		const group1 = 'a', group2 = 'b';
+		const value1 = 1, value2 = 2, value3 = 3;
+		let   source = [
+					{ key: group1, value: value1 },
+					{ key: group1, value: value2 },
+					{ key: group2, value: value3 },
+				];
+
+		let grouped = collections.groupBy(source, x => x.key);
+
+		// Group 1
+		assert.equal(grouped[group1].length, 2);
+		assert.equal(grouped[group1][0].value, value1);
+		assert.equal(grouped[group1][1].value, value2);
+
+		// Group 2
+		assert.equal(grouped[group2].length, 1);
+		assert.equal(grouped[group2][0].value, value3);
+	});
+
+	test('insert', () => {
+
+		const expected = 'value', hashFn = x => x.toString();
+
+		let into = collections.createStringDictionary();
+		collections.insert(into, expected, hashFn);
+
+		assert.equal(into[expected], expected);
 	});
 
 	test('remove', () => {