Clarify heavy vs light Isolates

[DartBot]

This issue was originally filed by mn...@gmail.com

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The standard library defines the Isolate class and talks about heavy and light Isolates. I've been unable to find any details about the two kinds of isolates in the spec. It could be valuable to clarify how a Dart implementation may handle them. For example, if a light isolate behaves sluggishly, is the runtime system allowed to silently upgrade it to a heavy isolate to preserve responsiveness in the main thread?

[DartBot]

This comment was originally written by drfibonacci@google.com

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Added Area-Language, Triaged labels.

[DartBot]

This comment was originally written by drfibonacci@google.com

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Removed Type-Defect label.
Added Type-Enhancement label.

[gbracha]

Removed Area-Language label.
Added Area-Library label.

[sigmundch]

Added Isolates label.

[sigmundch]

Added Duplicate label.
Marked as being merged into #3048.