Don't create expando object literals in TS (microsoft#26525)

Previously, getWidenedTypedFromJSPropertyAssignment was not called for
Typescript code. Since property assignments on functions, it is. That
meant that property assignments would incorrectly create a JS container
for empty object literals in a property assignment, even in Typescript:

```ts
const one = () => 1
one.p = {}
one.p.q = {} // should not work in Typescript!
```

Now empty object literals never create expando objects in Typescript,
because getJSExpandoObjectType requires the declaration to be in a JS
file.

Author: sandersn

src/compiler/checker.ts

@@ -4795,7 +4795,7 @@ namespace ts {
         }
 
         function getJSExpandoObjectType(decl: Node, symbol: Symbol, init: Expression | undefined): Type | undefined {
-            if (!init || !isObjectLiteralExpression(init) || init.properties.length) {
+            if (!isInJavaScriptFile(decl) || !init || !isObjectLiteralExpression(init) || init.properties.length) {
                 return undefined;
             }
             const exports = createSymbolTable();

tests/baselines/reference/typeFromPropertyAssignment30.js

@@ -0,0 +1,15 @@
+//// [typeFromPropertyAssignment30.ts]
+interface Combo {
+    (): number;
+    p?: { [s: string]: number };
+}
+const c: Combo = () => 1
+// should not be an expando object, but contextually typed by Combo.p
+c.p = {}
+
+
+
+//// [typeFromPropertyAssignment30.js]
+var c = function () { return 1; };
+// should not be an expando object, but contextually typed by Combo.p
+c.p = {};

tests/baselines/reference/typeFromPropertyAssignment30.symbols

@@ -0,0 +1,20 @@
+=== tests/cases/conformance/salsa/typeFromPropertyAssignment30.ts ===
+interface Combo {
+>Combo : Symbol(Combo, Decl(typeFromPropertyAssignment30.ts, 0, 0))
+
+    (): number;
+    p?: { [s: string]: number };
+>p : Symbol(Combo.p, Decl(typeFromPropertyAssignment30.ts, 1, 15))
+>s : Symbol(s, Decl(typeFromPropertyAssignment30.ts, 2, 11))
+}
+const c: Combo = () => 1
+>c : Symbol(c, Decl(typeFromPropertyAssignment30.ts, 4, 5), Decl(typeFromPropertyAssignment30.ts, 4, 24))
+>Combo : Symbol(Combo, Decl(typeFromPropertyAssignment30.ts, 0, 0))
+
+// should not be an expando object, but contextually typed by Combo.p
+c.p = {}
+>c.p : Symbol(Combo.p, Decl(typeFromPropertyAssignment30.ts, 1, 15))
+>c : Symbol(c, Decl(typeFromPropertyAssignment30.ts, 4, 5), Decl(typeFromPropertyAssignment30.ts, 4, 24))
+>p : Symbol(Combo.p, Decl(typeFromPropertyAssignment30.ts, 1, 15))
+
+

tests/baselines/reference/typeFromPropertyAssignment30.types

@@ -0,0 +1,21 @@
+=== tests/cases/conformance/salsa/typeFromPropertyAssignment30.ts ===
+interface Combo {
+    (): number;
+    p?: { [s: string]: number };
+>p : { [s: string]: number; }
+>s : string
+}
+const c: Combo = () => 1
+>c : Combo
+>() => 1 : { (): number; p: {}; }
+>1 : 1
+
+// should not be an expando object, but contextually typed by Combo.p
+c.p = {}
+>c.p = {} : {}
+>c.p : { [s: string]: number; }
+>c : Combo
+>p : { [s: string]: number; }
+>{} : {}
+
+

tests/cases/conformance/salsa/typeFromPropertyAssignment30.ts

@@ -0,0 +1,8 @@
+interface Combo {
+    (): number;
+    p?: { [s: string]: number };
+}
+const c: Combo = () => 1
+// should not be an expando object, but contextually typed by Combo.p
+c.p = {}
+