[AIRFLOW-3887] Downgrade dagre-d3 to 0.4.18

[astahlman]

Make sure you have checked all steps below.

Jira

• [ X ] My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-3887
  • In case you are fixing a typo in the documentation you can prepend your commit with [AIRFLOW-XXX], code changes always need a Jira issue.

Description

• [ X ] Here are some details about my PR, including screenshots of any UI changes:

dagre-d3 v0.6.3 has a bug that causes this Javascript error when loading
the Graph View:

TypeError: previousPaths.merge is not a function

The bug fix [1] has been merged to master, but hasn't been released to
npm yet. This change temporarily downgrades our version of dagre-d3
until dagre-d3 v0.6.4 is released [2]

I also fixed a bug I encountered in the compile_assets.sh where the
script would fail if the directory airflow/www/static/dist/ exists but
is empty.

[1] dagrejs/dagre-d3#350
[2] https://github.com/dagrejs/dagre-d3/blob/5450627790ff42012ef50cef6b0e220199ae4fbe/package.json#L3

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• [ X ] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• [ X ] Passes flake8

[astahlman]

Before

After

Not sure about that Started: Invalid date, but it doesn't seem to be related to this change.

[astahlman]

cc @feng-tao

[feng-tao]

LGTM