No compound assignement operators

Summary:
Addresses a performance regression introduced by automatic linting: Compound assignment operators are much slower than keeping assignment separate on `let` bindings in certain versions of v8.

This reverts the relevant changes and configures eslint to *disallow* these operators explicitly in `metro-source-map`.

Reviewed By: mjesun

Differential Revision: D6520485

fbshipit-source-id: 16f35f5cd691ce6b1924480cbc30fbaa1275f730

packages/metro-source-map/.eslintrc

@@ -0,0 +1,8 @@
+{
+  "rules": {
+    "operator-assignment": ["error", "never"]
+  },
+  "env": {
+    "node": true
+  }
+}

packages/metro-source-map/src/encode.js

@@ -167,11 +167,11 @@ function encode(value: number, buffer: Buffer, position: number): number {
   let digit;
   do {
     digit = vlq & VLQ_BASE_MASK;
-    vlq >>>= VLQ_BASE_SHIFT;
+    vlq = vlq >>> VLQ_BASE_SHIFT;
     if (vlq > 0) {
       // There are still more digits in this value, so we must make sure the
       // continuation bit is marked.
-      digit |= VLQ_CONTINUATION_BIT;
+      digit = digit | VLQ_CONTINUATION_BIT;
     }
     buffer[position++] = CHAR_MAP[digit];
   } while (vlq > 0);

packages/metro-source-map/src/source-map.js

@@ -56,7 +56,7 @@ function fromRawMappings(
       );
     }
 
-    carryOver += countLines(code);
+    carryOver = carryOver + countLines(code);
   }
 
   return generator;