Fix column offset calculation in sectioned source maps

Summary:
Fixes a bug in `metro-source-map`'s source map consumer implementation, where positions inside a section with a nonzero column offset would be incorrectly shifted during lookup.

If `A` represents characters mapped by one section and `B` represents characters mapped by another section:
```
// Generated code

1 | AAAAAAAAAA
2 | AAAAABBBBB
         ^
        (i)
3 | BBBBBBBBBB
          ^
         (ii)
```

Then in order to perform a look up in the source map for section B, the positions marked above should be translated to:
```
// "B" part of generated code

1 | BBBBB
    ^
   (i)
2 | BBBBBBBBBB
          ^
         (ii)
```

But with the bug we instead translated them to:

```
// "B" part of generated code

1 | BBBBB
    ^
   (i - correct)
2 | BBBBBBBBBB
     ^
    (ii - incorrect)
```

Reviewed By: MichaReiser

Differential Revision: D26166640

fbshipit-source-id: db510bf7e20b9eb9e67e06cc3dedcab95410b79b

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

@@ -10,7 +10,7 @@
 
 'use strict';
 
-const {add, neg} = require('ob1');
+const {add, neg, add0, add1} = require('ob1');
 
 import type {GeneratedOffset} from './types.flow';
 import type {Number0, Number1} from 'ob1';
@@ -36,9 +36,15 @@ function subtractOffsetFromPosition<
     ...
   },
 >(pos: T, offset: GeneratedOffset): T {
+  if (pos.line === add1(offset.lines)) {
+    return shiftPositionByOffset(pos, {
+      lines: neg(offset.lines),
+      columns: neg(offset.columns),
+    });
+  }
   return shiftPositionByOffset(pos, {
     lines: neg(offset.lines),
-    columns: neg(offset.columns),
+    columns: add0(0),
   });
 }
 

packages/metro-source-map/src/__tests__/Consumer-test.js

@@ -321,6 +321,68 @@ describe('indexed (sectioned) maps', () => {
       `);
     });
 
+    test('column offset only applies to first line of section', () => {
+      const consumer = new Consumer({
+        version: 3,
+        sections: [
+          {
+            offset: {line: 5, column: 1000},
+            map: {
+              version: 3,
+              names: ['section0_name0', 'section0_name1'],
+              sources: ['section0_source0', 'section0_source1'],
+              mappings: 'AAEEA,G;ECAAC,C;C',
+            },
+          },
+        ],
+      });
+      expect(consumer.originalPositionFor({line: add1(5), column: add0(1002)}))
+        .toMatchInlineSnapshot(`
+        Object {
+          "column": 2,
+          "line": 3,
+          "name": "section0_name0",
+          "source": "section0_source0",
+        }
+      `);
+      expect(consumer.originalPositionFor({line: add1(5), column: add0(1003)}))
+        .toMatchInlineSnapshot(`
+        Object {
+          "column": null,
+          "line": null,
+          "name": null,
+          "source": null,
+        }
+      `);
+      expect(consumer.originalPositionFor({line: add1(6), column: add0(2)}))
+        .toMatchInlineSnapshot(`
+        Object {
+          "column": 2,
+          "line": 3,
+          "name": "section0_name1",
+          "source": "section0_source1",
+        }
+      `);
+      expect(consumer.originalPositionFor({line: add1(6), column: add0(3)}))
+        .toMatchInlineSnapshot(`
+        Object {
+          "column": null,
+          "line": null,
+          "name": null,
+          "source": null,
+        }
+      `);
+      expect(consumer.originalPositionFor({line: add1(6), column: add0(1002)}))
+        .toMatchInlineSnapshot(`
+        Object {
+          "column": null,
+          "line": null,
+          "name": null,
+          "source": null,
+        }
+      `);
+    });
+
     test('unmapped regions', () => {
       const consumer = new Consumer({
         version: 3,