Emit defineProperty calls before param prop assignments

Note that I restricted this to --useDefineForClassFields is true.
Nothing changes when it's off. I think this is the correct fix for a
patch release.

However, in principal there's nothing wrong with moving parameter
property initialisation after property declaration initialisation. It
would be Extremely Bad and Wrong to rely on this working:

```ts
class C {
  p = this.q // what is q?
  constructor(public q: number) { }
}
```

But today it does, and probably somebody relies on it without knowing.

src/compiler/transformers/classFields.ts

@@ -10,9 +10,9 @@ namespace ts {
     /**
      * Transforms ECMAScript Class Syntax.
      * TypeScript parameter property syntax is transformed in the TypeScript transformer.
-     * For now, this transforms public field declarations using TypeScript class semantics
-     * (where the declarations get elided and initializers are transformed as assignments in the constructor).
-     * Eventually, this transform will change to the ECMAScript semantics (with Object.defineProperty).
+     * For now, this transforms public field declarations using TypeScript class semantics,
+     * where declarations are elided and initializers are transformed as assignments in the constructor.
+     * When --useDefineForClassFields is on, this transforms to ECMAScript semantics, with Object.defineProperty.
      */
     export function transformClassFields(context: TransformationContext) {
         const {
@@ -294,7 +294,8 @@ namespace ts {
         }
 
         function transformConstructorBody(node: ClassDeclaration | ClassExpression, constructor: ConstructorDeclaration | undefined, isDerivedClass: boolean) {
-            const properties = getProperties(node, /*requireInitializer*/ !context.getCompilerOptions().useDefineForClassFields, /*isStatic*/ false);
+            const useDefineForClassFields = context.getCompilerOptions().useDefineForClassFields;
+            const properties = getProperties(node, /*requireInitializer*/ !useDefineForClassFields, /*isStatic*/ false);
 
             // Only generate synthetic constructor when there are property initializers to move.
             if (!constructor && !some(properties)) {
@@ -325,6 +326,9 @@ namespace ts {
             if (constructor) {
                 indexOfFirstStatement = addPrologueDirectivesAndInitialSuperCall(constructor, statements, visitor);
             }
+            if (useDefineForClassFields) {
+                addPropertyStatements(statements, properties, createThis());
+            }
 
             // Add the property initializers. Transforms this:
             //
@@ -336,7 +340,7 @@ namespace ts {
             //      this.x = 1;
             //  }
             //
-            if (constructor && constructor.body) {
+            if (constructor?.body) {
                 let parameterPropertyDeclarationCount = 0;
                 for (let i = indexOfFirstStatement; i < constructor.body.statements.length; i++) {
                     if (isParameterPropertyDeclaration(getOriginalNode(constructor.body.statements[i]), constructor)) {
@@ -351,7 +355,9 @@ namespace ts {
                     indexOfFirstStatement += parameterPropertyDeclarationCount;
                 }
             }
-            addPropertyStatements(statements, properties, createThis());
+            if (!useDefineForClassFields) {
+                addPropertyStatements(statements, properties, createThis());
+            }
 
             // Add existing statements, skipping the initial super call.
             if (constructor) {

tests/baselines/reference/definePropertyES5.js

@@ -6,6 +6,7 @@ class A {
     ["computed"] = 13
     ;[x] = 14
     m() { }
+    constructor(public readonly y: number) { }
 }
 
 
@@ -13,7 +14,13 @@ class A {
 var _a;
 var x = "p";
 var A = /** @class */ (function () {
-    function A() {
+    function A(y) {
+        Object.defineProperty(this, "y", {
+            enumerable: true,
+            configurable: true,
+            writable: true,
+            value: void 0
+        });
         Object.defineProperty(this, "a", {
             enumerable: true,
             configurable: true,
@@ -38,6 +45,7 @@ var A = /** @class */ (function () {
             writable: true,
             value: 14
         });
+        this.y = y;
     }
     Object.defineProperty(A.prototype, "m", {
         enumerable: false,

tests/baselines/reference/definePropertyES5.symbols

@@ -21,5 +21,8 @@ class A {
 
     m() { }
 >m : Symbol(A.m, Decl(definePropertyES5.ts, 5, 13))
+
+    constructor(public readonly y: number) { }
+>y : Symbol(A.y, Decl(definePropertyES5.ts, 7, 16))
 }
 

tests/baselines/reference/definePropertyES5.types

@@ -25,5 +25,8 @@ class A {
 
     m() { }
 >m : () => void
+
+    constructor(public readonly y: number) { }
+>y : number
 }
 

tests/baselines/reference/definePropertyESNext.js

@@ -0,0 +1,43 @@
+//// [definePropertyESNext.ts]
+var x: "p" = "p"
+class A {
+    a = 12
+    b
+    ["computed"] = 13
+    ;[x] = 14
+    m() { }
+    constructor(public readonly y: number) { }
+}
+class B {
+}
+class C extends B {
+    z = 1
+    constructor(public ka: number) {
+        super()
+    }
+}
+
+
+//// [definePropertyESNext.js]
+var x = "p";
+class A {
+    y;
+    a = 12;
+    b;
+    ["computed"] = 13;
+    [x] = 14;
+    m() { }
+    constructor(y) {
+        this.y = y;
+    }
+}
+class B {
+}
+class C extends B {
+    ka;
+    z = 1;
+    constructor(ka) {
+        super();
+        this.ka = ka;
+    }
+}

tests/baselines/reference/definePropertyESNext.symbols

@@ -0,0 +1,45 @@
+=== tests/cases/conformance/classes/propertyMemberDeclarations/definePropertyESNext.ts ===
+var x: "p" = "p"
+>x : Symbol(x, Decl(definePropertyESNext.ts, 0, 3))
+
+class A {
+>A : Symbol(A, Decl(definePropertyESNext.ts, 0, 16))
+
+    a = 12
+>a : Symbol(A.a, Decl(definePropertyESNext.ts, 1, 9))
+
+    b
+>b : Symbol(A.b, Decl(definePropertyESNext.ts, 2, 10))
+
+    ["computed"] = 13
+>["computed"] : Symbol(A["computed"], Decl(definePropertyESNext.ts, 3, 5))
+>"computed" : Symbol(A["computed"], Decl(definePropertyESNext.ts, 3, 5))
+
+    ;[x] = 14
+>[x] : Symbol(A[x], Decl(definePropertyESNext.ts, 5, 5))
+>x : Symbol(x, Decl(definePropertyESNext.ts, 0, 3))
+
+    m() { }
+>m : Symbol(A.m, Decl(definePropertyESNext.ts, 5, 13))
+
+    constructor(public readonly y: number) { }
+>y : Symbol(A.y, Decl(definePropertyESNext.ts, 7, 16))
+}
+class B {
+>B : Symbol(B, Decl(definePropertyESNext.ts, 8, 1))
+}
+class C extends B {
+>C : Symbol(C, Decl(definePropertyESNext.ts, 10, 1))
+>B : Symbol(B, Decl(definePropertyESNext.ts, 8, 1))
+
+    z = 1
+>z : Symbol(C.z, Decl(definePropertyESNext.ts, 11, 19))
+
+    constructor(public ka: number) {
+>ka : Symbol(C.ka, Decl(definePropertyESNext.ts, 13, 16))
+
+        super()
+>super : Symbol(B, Decl(definePropertyESNext.ts, 8, 1))
+    }
+}
+

tests/baselines/reference/definePropertyESNext.types

@@ -0,0 +1,51 @@
+=== tests/cases/conformance/classes/propertyMemberDeclarations/definePropertyESNext.ts ===
+var x: "p" = "p"
+>x : "p"
+>"p" : "p"
+
+class A {
+>A : A
+
+    a = 12
+>a : number
+>12 : 12
+
+    b
+>b : any
+
+    ["computed"] = 13
+>["computed"] : number
+>"computed" : "computed"
+>13 : 13
+
+    ;[x] = 14
+>[x] : number
+>x : "p"
+>14 : 14
+
+    m() { }
+>m : () => void
+
+    constructor(public readonly y: number) { }
+>y : number
+}
+class B {
+>B : B
+}
+class C extends B {
+>C : C
+>B : B
+
+    z = 1
+>z : number
+>1 : 1
+
+    constructor(public ka: number) {
+>ka : number
+
+        super()
+>super() : void
+>super : typeof B
+    }
+}
+

tests/cases/conformance/classes/propertyMemberDeclarations/definePropertyES5.ts

@@ -7,4 +7,5 @@ class A {
     ["computed"] = 13
     ;[x] = 14
     m() { }
+    constructor(public readonly y: number) { }
 }

tests/cases/conformance/classes/propertyMemberDeclarations/definePropertyESNext.ts

@@ -0,0 +1,19 @@
+// @target: esnext
+// @useDefineForClassFields: true
+var x: "p" = "p"
+class A {
+    a = 12
+    b
+    ["computed"] = 13
+    ;[x] = 14
+    m() { }
+    constructor(public readonly y: number) { }
+}
+class B {
+}
+class C extends B {
+    z = 1
+    constructor(public ka: number) {
+        super()
+    }
+}