node.loc.source is not being handled correctly.

[citrizon]

Motivation

When i am stringifying an AST that is ES-Tree compliant, the locations (through "loc" property) works but the sources are not matching. The "loc" property also has a nested property called "source" which is a string. When the AST is being generated, astring uses the generated file name inside "file" property inside the sourcemap instead of checking the "source" property inside the "loc" object.

Expected behavior

Testing AST:

{
  "type": "Program",
  "sourceType": "script",
  "body": [
    {
      "type": "VariableDeclaration",
      "kind": "let",
      "declarations": [
        {
          "type": "VariableDeclarator",
          "id": {
            "type": "Identifier",
            "name": "hi",
            "loc": {
              "start": {
                "line": 1,
                "column": 4
              },
              "end": {
                "line": 1,
                "column": 6
              },
              "source": "test1.js"
            }
          },
          "init": {
            "type": "Literal",
            "value": 1,
            "loc": {
              "start": {
                "line": 1,
                "column": 9
              },
              "end": {
                "line": 1,
                "column": 10
              },
              "source": "test1.js"
            }
          },
          "loc": {
            "start": {
              "line": 1,
              "column": 4
            },
            "end": {
              "line": 1,
              "column": 10
            },
            "source": "test1.js"
          }
        }
      ],
      "loc": {
        "start": {
          "line": 1,
          "column": 0
        },
        "end": {
          "line": 1,
          "column": 11
        },
        "source": "test1.js"
      }
    },
    {
      "type": "VariableDeclaration",
      "kind": "let",
      "declarations": [
        {
          "type": "VariableDeclarator",
          "id": {
            "type": "Identifier",
            "name": "hey",
            "loc": {
              "start": {
                "line": 1,
                "column": 4
              },
              "end": {
                "line": 1,
                "column": 7
              },
              "source": "test2.js"
            }
          },
          "init": {
            "type": "Literal",
            "value": 2,
            "loc": {
              "start": {
                "line": 1,
                "column": 10
              },
              "end": {
                "line": 1,
                "column": 11
              },
              "source": "test2.js"
            }
          },
          "loc": {
            "start": {
              "line": 1,
              "column": 4
            },
            "end": {
              "line": 1,
              "column": 11
            },
            "source": "test2.js"
          }
        }
      ],
      "loc": {
        "start": {
          "line": 1,
          "column": 0
        },
        "end": {
          "line": 1,
          "column": 12
        },
        "source": "test2.js"
      }
    }
  ],
  "loc": {
    "start": {
      "line": 1,
      "column": 0
    },
    "end": {
      "line": 2,
      "column": 12
    }
  }
}

Expected SourceMap:

{
  version: 3,
  sources: [  'test1.js', 'test2.js' ],
  names: [ 'hi', 'hey' ],
  mappings: 'IAAIA,KAAK;ICALC,MAAM',
  file: 'generated.js',
  sourceRoot: './'
}

Actual behavior

astring does not use the source names inside the loc property and so it makes the sourcemap incorrect.
This is the result of actual behavior:

{
  version: 3,
  sources: [ 'generated.js' ],
  names: [ 'hi', 'hey' ],
  mappings: 'IAAIA,KAAK;IAALC,MAAM',
  file: 'generated.js',
  sourceRoot: './'
}

Fix:

Literally just add mapping.source = node.loc.source; to astring.js at new line 1164. I've already created a small pull request for this: #732