chore(build): upgrade to TypeScript 3.2.1

[raymondfeng]

See https://blogs.msdn.microsoft.com/typescript/2018/11/29/announcing-typescript-3-2/

Checklist

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

[bajtos]

Hmm, are you sure this change is preserving current behavior?

A test case to consider:

cast({and: [{x: 1}, {y: 2}], z: 3})

Before your change, cast returns

{and: [{x: 1}, {y: 2}]}

After your change, cast returns

{and: [{x: 1}, {y: 2}], z: 3}

I am typing these examples from my head based on how I understand the code. I may be wrong.

@raymondfeng please take a second look at this change.

If the new implementation of cast is just an identity function (returning the input without any change), then I think we should mark cast as deprecated (at minimum in TSDocs; ideally also at runtime using depd) and remove it in the next major version.

This does not necessarily has to happen in this pull request, but it would be great to make those changes soon, otherwise we are increasing our technical debt.

Thoughts?

[bajtos]

Why underscore? Can we use fields or perhaps allFields/filterFields to better distinguish from field name used later?

[bajtos]

Nice!

I am proposing to make the following additional changes while upgrading to TS 3.2:

1. Enable strictBindCallApply (docs)

2. Leverage Configuration inheritance via node_modules packages (docs) to simplify tsconfig.build.json in our packages & examples, see e.g. packages/core and examples/todo.

   I think we should use the following form everywhere:

   "extends": "@loopback/build/config/tsconfig.common.json",
   

[bajtos]

Considering that our build is failing without some of these changes, I am ok to land this pull request as-is in order to fix our failing builds.

I wish the change-set could have been smaller though, I am not confident that this patch is fully backwards compatible 😟