Implement non-null assertions (#443)

tests/compiler/nonNullAssertion.ts

@@ -0,0 +1,55 @@
+import "allocator/arena";
+
+export function testVar(n: Error | null): Error {
+  return n!;
+}
+
+class Foo {
+  bar: Foo | null;
+  baz: (() => Foo | null) | null;
+}
+
+export function testObj(foo: Foo | null): Foo | null {
+  return foo!.bar;
+}
+
+export function testProp(foo: Foo): Foo {
+  return foo.bar!;
+}
+
+export function testArr(foo: Array<Foo> | null): Foo {
+  return foo![0];
+}
+
+export function testElem(foo: Array<Foo | null>): Foo {
+  return foo[0]!;
+}
+
+export function testAll(foo: Array<Foo | null> | null): Foo {
+  return foo![0]!.bar!;
+}
+
+export function testAll2(foo: Array<Foo | null> | null): Foo {
+  return foo!![0]!!!.bar!!!!;
+}
+
+export function testFn(fn: (() => Foo | null) | null): Foo | null {
+  return fn!();
+}
+
+export function testFn2(fn: (() => Foo | null) | null): Foo | null {
+  var fn2 = fn!;
+  return fn2();
+}
+
+export function testRet(fn: (() => Foo | null) | null): Foo {
+  return fn!()!;
+}
+
+export function testObjFn(foo: Foo): Foo | null {
+  return foo.baz!();
+}
+
+export function testObjRet(foo: Foo): Foo {
+  return foo.baz!()!;
+}