[browser][CoreCLR][trimmed] RuntimeBinder array indexer tests throw NullReferenceException

[lewing]

Description

Two System.Dynamic.Runtime.Tests array-indexer tests throw NullReferenceException inside Microsoft.CSharp.RuntimeBinder.ExpressionTreeCallRewriter when the browser-Wasm CoreCLR test app is aggressively trimmed.

Affected methods:

ManagedTests.DynamicCSharp.Conformance.dynamic.dynamicType.basic.array005.array005.Program.DynamicCSharpRunTest
ManagedTests.DynamicCSharp.Conformance.dynamic.dynamicType.basic.array006.array006.Program.DynamicCSharpRunTest

Found while bringing up the broad trimmed ReadyToRun library lane from #133193. The same failures reproduce with CoreCLR trimming and ReadyToRun disabled, so they are trimming/RuntimeBinder failures rather than R2R codegen failures.

Reproduction Steps

Build and run System.Dynamic.Runtime.Tests for browser-Wasm CoreCLR with:

EnableAggressiveTrimming=true
TestTrimming=true
RuntimeFlavor=CoreCLR
Configuration=Release

Expected behavior

Both dynamic array-indexer tests pass as they do in the non-trimmed browser configuration.

Actual behavior

System.NullReferenceException
   at Microsoft.CSharp.RuntimeBinder.ExpressionTreeCallRewriter.Rewrite(ExprBinOp, Expression[])
   at Microsoft.CSharp.RuntimeBinder.RuntimeBinder.BindCore(...)

Regression?

Unknown. Broad CoreCLR browser trimming coverage is newly enabled.

Known Workarounds

Temporarily exclude only these two methods when IsBuiltWithAggressiveTrimming && IsBrowser. Non-trimmed browser and other configurations remain enabled.

Configuration

Browser-Wasm CoreCLR Release with aggressive trimming, Chrome.

Other information

Re-enable when the RuntimeBinder expression-tree rewrite path works after trimming or the required metadata is explicitly preserved.

Note

This issue was generated with GitHub Copilot assistance and reviewed as part of the #133193 bring-up.

[dotnet-policy-service]

Tagging subscribers to 'arch-wasm': @lewing, @pavelsavara
See info in area-owners.md if you want to be subscribed.

[dotnet-policy-service]

Tagging subscribers to this area: @phil-allen-msft, @333fred
See info in area-owners.md if you want to be subscribed.

[lewing]

This is bookkeeping for the libraries test stand up.