Handle "simple" (missing origin) source mappings in minifier output

Summary:
Fixes #927

Terser, unlike the previous default minifier UglifyES, produces source maps whose entries may contain "simple mappings" - that is entries with a generated line+col but no original location.

When Metro merges source maps to serialize minified output, it doesn't correctly handle these entries. Instead it incorrectly detects them as complete mappings and produces a merged source map with `-1` origin line numbers in place of simple mappings.

Some tools will reject the whole source map (which is otherwise correct) based on the appearance of a negative line number.

The issue comes down to the conversion of raw mappings to `BabelSourceMapSegments` in `toBabelSegments`, which currently generates entries for simple mappings with `null` `origin` *properties*, such as:

```
{
  generated: {
    column: 2,
    line: 1
  },
  origin: {
    column: null,
    line: null
  },
  // ...
}
```

According to the Flow type for `BabelSourceMapSegments`, that's incorrect. `origin` itself is optional and should be missing - the properties of `origin` are not nullable.

Based on the presence of the `origin` property in that object, `toSegmentTuple` generates a 4-tuple and we go on to add a "source mapping" rather than a "simple mapping".

This fixes `toBabelSegments` to omit `origin` unless `line` and `column` are both populated. The final source map output is then correct.

Differential Revision: D43351987

fbshipit-source-id: 5ba5e9edc6913d1feaf7a6b1827ad43e579fbada

packages/metro-source-map/src/__tests__/__snapshots__/source-map-test.js.snap

@@ -8,10 +8,6 @@ Array [
       "line": 1,
     },
     "name": null,
-    "original": Object {
-      "column": null,
-      "line": null,
-    },
     "source": null,
   },
   Object {
@@ -56,10 +52,6 @@ Array [
       "line": 12,
     },
     "name": null,
-    "original": Object {
-      "column": null,
-      "line": null,
-    },
     "source": null,
   },
   Object {
@@ -80,10 +72,6 @@ Array [
       "line": 25,
     },
     "name": null,
-    "original": Object {
-      "column": null,
-      "line": null,
-    },
     "source": null,
   },
   Object {

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

@@ -206,18 +206,29 @@ function toBabelSegments(
   const rawMappings: Array<BabelSourceMapSegment> = [];
 
   new SourceMap.SourceMapConsumer(sourceMap).eachMapping(map => {
-    rawMappings.push({
-      generated: {
-        line: map.generatedLine,
-        column: map.generatedColumn,
-      },
-      original: {
-        line: map.originalLine,
-        column: map.originalColumn,
-      },
-      source: map.source,
-      name: map.name,
-    });
+    rawMappings.push(
+      map.originalLine == null || map.originalColumn == null
+        ? {
+            generated: {
+              line: map.generatedLine,
+              column: map.generatedColumn,
+            },
+            source: map.source,
+            name: map.name,
+          }
+        : {
+            generated: {
+              line: map.generatedLine,
+              column: map.generatedColumn,
+            },
+            original: {
+              line: map.originalLine,
+              column: map.originalColumn,
+            },
+            source: map.source,
+            name: map.name,
+          },
+    );
   });
 
   return rawMappings;