initial pass at equality as member type guards

tests/cases/conformance/expressions/typeGuards/typeGuardByEqualityCheck.ts

@@ -0,0 +1,44 @@
+interface Discriminator {
+	_discriminator: void;
+}
+
+interface FooDiscriminator extends Discriminator {
+	_foo: void;
+}
+
+interface BarDiscriminator extends Discriminator {
+	_bar: void;
+}
+
+interface BaseNode {
+	kind: Discriminator;
+}
+
+interface FooNode extends BaseNode {
+	kind: FooDiscriminator;
+	foo: string;
+}
+
+interface BarNode extends BaseNode {
+	kind: BarDiscriminator;
+	bar: string;
+}
+
+let a: FooDiscriminator;
+let x: FooNode | BarNode;
+
+if (x.kind === a) {
+	x.foo = "yay!";
+}
+else {
+	x; // Not narrowed at present
+}
+
+let z: {
+	value: string;
+	item: FooNode | BarNode;
+}
+if (z.item.kind === a) {
+	z.item.foo = "cool!";
+	z.value = "yes";
+}