Reduce --release apk and ipa sizes

[eseidelGoogle]

We've had a bunch of issues related to this, most directly #12456 as another. Attempting to create an issue which we can drive to action.

Android Instant Apps #7614 will require a < 4MB initial .apk size (which doesn't have to include the Flutter engine perhaps, but should).

Our current "hello_world" apk size at time of writing is:
basic_material_app_android__compile release_size_bytes is 7907K
basic_material_app_ios__compile release_size_bytes is 9365K

My understanding is their is a ton of low hanging fruit here.

• @mraleph has identified there is ~1mb of extra binary size in the dart runtime currently due to including observatory assets.
• @chinmaygarde has previously stated that he believes there is about 1mb (compressed) of ICU data size to save with some compile flag changes.
• It's possible we could even find a copy of ICU on Android or iOS which we could use directly (via GMSCore or other installed assets, we could even imagine having a fall-back path of locations to look). We investigated this in both Use system ICU on iOS #392 and Use system ICU on Android #394 but things may have changed.

I'm sure there is a ton more we could do here with some investigation. This was our 3rd most requested work area in our most recent user survey.

FYI @cbracken @Hixie

[ohir]

Support for Vector Graphics resources precompiled to some compact representation is crucial for the apk size, too. See #1831 discussion.

There are tools in AndroidStudio to convert .svg files to the xml based VectorDrawable.
This xml can be a source file compiled for more compact format, e.g. one sketched at
https://godoc.org/golang.org/x/exp/shiny/iconvg.

[dnfield]

@ohir while asset compression/refactoring can certainly reduce APK/IPA size, I think this issue has more to do with "base" binary size for a bare minimum flutter app.

[chinmaygarde]

Update: Was investigating this some more today. We only include observatory assets in the Debug and Profile runtime modes. However, vmservice sources are included in the release mode even though we will never initialize the same in that mode.

[mraleph]

@chinmaygarde I am working on removing vmservice in release mode in dart-lang/sdk#32834

[saeid-ir]

Also mention that if you look at Storage usage of an app after installing the application in settings, its getting two or tree time more storage than the same app written in native.
I think its too heavy in comparison.