Question

Can someone explain what the patch keyword does? For example, in
math_patch.dart I see

patch num pow(num x, num exponent) => MathNatives.pow(x, exponent);
patch double atan2(num a, num b) => MathNatives.atan2(a, b);
patch double sin(num x) => MathNatives.sin(x);
patch double cos(num x) => MathNatives.cos(x);

What does this mean? What are _patch.dart files for?

Answer 1

The patch mechanism is used internally (and is only available
internally, not to end users) to provide different implementations of
core library functionality.

For the math library that you have below, the platform independent
library source in lib/math declares these methods as external.
external methods get their implementation from a patch file. There
is a patch file in the VM in runtime/lib/math_patch.dart, which
supplies the implementation for the VM and there is a patch file in
the dart2js compiler in
lib/compiler/implementation/lib/math_patch.dart, which supplies the
dart2js implementation.

The external keyword is understood by the analyzer and doing it this
way allows only the shared part to be in the SDK and be understood by
the tools. That means that the SDK can have lib/math instead of having
lib/math/runtime and lib/math/dart2js, which makes the SDK cleaner and
easier to understand.